1. Field of the Invention
The present invention relates to a method and apparatus for the non-destructive measurement of the dispersion zero along an optical fiber.
2. Description of Related Art
Traditional measurements of the chromatic dispersion of an optical fiber provides its integral value over a fiber span. One of the more common methods of doing so is to measure the group delay of the span as a function of wavelength. Dispersion is derived from the relationship ##EQU1## where c is the speed of light, .beta. is the second derivative of .beta.(.omega.) the frequency dependent propagation constant in the fiber with respect to .omega., the angular frequency, and .lambda. is the wavelength. Since the group delay is proportional to the first derivative of .beta.(.omega.) with respect to .omega., D(.lambda.) can be obtained by taking the derivative of the group delay with respect to .omega.. However, this measurement only provides the average dispersion of the fiber as noted above.
Detailed knowledge of the actual dispersion along a fiber span, rather than its average, becomes more important as lightwave systems move towards the use of higher optical powers, longer span lengths, higher bit rates, and wavelength multiplexing. This is because controlled variations in dispersion along the fiber (called "dispersion management") has become a tool used by some system designers to suppress optical non-linearities and to manipulate these non-linearities to further enhance system performance. Without dispersion management, these optical non-linearities would cause spectral broading, increased pulse spreading, and mixing of multiplexed wavelength channels. While dispersion management is particularly useful in soliton systems and systems which span trans-oceanic distances, dispersion management is increasingly being applied to high performance, non-return-to-zero terrestrial systems as well. If dispersion management is to be effective one needs to know about unintentional variations in dispersion along individual lengths of fiber. Thus, there is interest in developing methods and apparatuses for the non-destructive measurement of the variation in dispersion along the length of an optical fiber. In any fiber, the dispersion passes through zero at some wavelength, .lambda..sub.0. This wavelength is referred to as the zero-dispersion wavelength. The value of .lambda..sub.0 along the fiber is a particularly convenient way to characterize dispersion along the fiber.
In dispersion-shifted fiber (DSF) the dispersion is known to vary as a function of location in the fiber. In one experiment using a ten kilometer ("km") length of DSF fiber, which was cut into four, 21/2 km segments, it was found that the average zero-dispersion wavelength of the segments varied by at least 1 nanometer ("nm"). See K. Inoue, "Four-wave mixing in an optical fiber in the zero-dispersion wavelength region", Journal of Lightwave Technology, v. 10, pp. 1553-1561, (1992).
Recently, a non-destructive, remote-measurement technique was used to determine the local zero-dispersion wavelength of an optical fiber by observing, on an optical time domain reflectometer, the gain that occurs at wavelengths slightly longer than the zero-dispersion wavelength (referred to as "modulation instability pulse amplification"), see S. Nishi and M. Saruwatari, "Technique for measuring the distributed zero-dispersion wavelength of optical fibers using pulse amplification caused by modulation instability", Electron. Lett., v. 31, pp. 225-226 (1995). The results achieved appear to indicate a wavelength resolution of several tenths of a nanometer, and a distance resolution of a kilometer or so. This technique offers the advantage of requiring access to only one end of the fiber. However, this new technique appears to require the accurate measurement of small (a few tenths of a decibel) changes in the level of a backscattered signal.